"You will be very thirsty after so much talking on the phone."
Translation:Vi tre soifos pro tiom da parolado ĉe la telefono.
… post telefoni tiom
… post tiom da telefonado
If you use telefoni the object is tiom an you do not need da. I think it would be better to put the object behind the infinitive.
If you want to give the information what you did so much you have to make a noun from the verb. This is normally done by the suffix -ado instead of -i.